Rotary drilling is one type of crushing work in which the actual crushing work is performed by a crushing tool, usually a rotary drill bit, a so-called crown, provided with three cone rollers. The cone rollers are journalled to the body of a rotary drill bit at a pitch of 120.degree.. The cone rollers are fitted with set hard metal studs having a round projecting end. The rotary drill bit is fastened to the end of a drill rod. The opposite end of the drill rod is fastened either to a neck element carried by the rotary head or to a transmitting drill rod with a screw connection. The crushing of rock is effected in rotary drilling by feeding a rotary drill bit perpendicularly against an object, such as a rock, being drilled while rotating it by means of the drill rods using a neck element carried by the rotary head. The rotary head is adapted by means of a feeding mechanism for movement inside a mast connected with a rotary drilling machine. The feeding mechanism comprises either a pinion/rack or a chain/motor transmission. The heavy-duty rotary drilling assemblies are electrically powered. The rotary head is usually provided with two D.C. motors and a necessary gearing whose output shaft is formed by a neck element to which the upper drill rod is screw-threaded.
In the presently used rotary drilling equipment, the neck element is secured axially immovably to the rotary head. Thus, the feeding force is transmitted directly thereby to the thrust bearings of a rotary head.
One of the most serious problems in equipment operating on the rotary drilling principle is irregular vibration caused on the one hand by the structure of a rotary drill bit and on the other hand by the characteristics of an object being drilled. First of all, the outer surface of the cone rollers of the rotary drill bit is provided with hard metal studs which project from the outer surface of cone wheels. The boss of cone rollers is set in reciprocating vertical motion which, in the presently used equipment, forces the drill rods and a rotary head at the end thereof into a corresponding motion. A second and perhaps more significant source of vibration are broken pieces of rock which cannot be immediately flushed away from under the rotary drill bit by flushing medium. The rotary drill bit will be forced to grind such pieces of rock to smaller size. During the crushing operation, these pieces of rock produce an upwardly-directed force on the rotary drill bit.
For the above reasons the presently used rotary drilling mechanisms involve a considerable number of drawbacks, the most important ones being listed hereinbelow:
The service life of a rotary drill bit is short since the bearings of cone rollers are rapidly fatigued and the hard metal studs wear down and fracture due to overload,
drilling capacity is not high since the fluctuation of feeding force is considerable (the feeding force needed for breaking rock is at times too low with upward-directed accelerations appearing in the mechanism),
substantial vibration which stresses the structures of a rotary drilling machine and a rotary drilling mechanism and causes deterioration in joints and feeding mechanism,
the bearings of a rotary head and electric motors are stressed,
the vibration tends to shift the entire rotary drilling machine resulting in a hazard of drill pipes bending, breaking and jamming in a bore hole, and
the working safety and conditions for the crew of a rotary drilling machine are poor, especially due to vibration.
FIG. 1 a curve indicating the fluctuation of the feeding force of presently used rotary drilling mechanisms relative to time. A straight line F.sub.S parallel to time axis indicates a feeding force required for the breaking effect in a given situation. A curve indicating a momentary feeding force fluctuates on either side of this line. The power peaks crossing above straight line F.sub.S put a particular stress on the bearings of a rotary drill bit with their service life determined by the peaks. The forces going below line F.sub.S are not sufficient for crushing a rock. The rapid fluctuations in feeding force cause vibration.
It is impossible to eliminate the above factors having an effect in rotary drilling and resulting on the one hand from a rotary drill bit and on the other hand from an object being drilled but their detrimental effects, both on rotary drilling operation and on rotary drilling equipment, can be overcome by applying a method of this invention in rotary drilling.